The deflection of the electron beam within a cathode ray tube of an oscilloscope must be periodically calibrated to the graticule on the faceplate to ensure that the graticule is accurately measuring the amplitude and duration of waveforms reproduced by the beam. Until the present invention, this calibration has largely been done manually at the factory. Typically, a calibration signal is applied as a deflection waveform input to the cathode ray tube to deflect the beam. An operator then visually determines the position of the beam relative to the graticule line and manually adjusts deflection amplifiers associated with the cathode ray tube to deflect the beam until it coincides with the graticule line.
This manual approach is expensive because it is slow and labor intensive. It is also relatively inaccurate. A single, manual calibration at the factory does not ensure calibration over the range of temperatures in which an oscilloscope operates. With programmable deflection amplifiers especially, the effect of temperature variations limits accuracy to 4%.
The present state of the art provides means whereby horizontal and vertical deflection parameters for the electron beam can be adjusted under software control. Calibration signal sources can be controlled by the same software. No capability, however, has been available to enable the software to properly set the deflection parameters relative to the graticule.
Several means have been developed to automatically detect the position of the electron beam relative to the faceplate of the cathode ray tube. For example, beam index display systems used in color televisions employ an indexing phosphor that emits ultraviolet light when struck by the electron beam as it is swept across the faceplate in a raster scan. This light is detected by a photodetector adjacent to the cathode ray tube, which generates a signal to synchronize the color video signals to the colored phosphors placed between indexing phosphors. Such systems are disclosed in U.S. Pat. No. 4,247,869 to Culter et al, U.S. Pat. No 2,790,107 to Bradley, U.S. Pat. No. 2,778,971 to Sunstein, and British Patent 822,017.
These systems, however, require the use of a special cathode ray tube having an indexing phosphor, and index to a phosphor inside the cathode ray tube, not a graticule line useful to an observer. Moreover, these systems do not provide a means for adjusting the deflection of the electron beam in response to the detection of its position.
Accordingly, a need remains for a method and apparatus for sensing beam position relative to the graticule and automatically calibrating the beam deflection parameters.